In order to maintain a sealing property for a long time in a mechanical seal serving as one example of sliding parts, contradictory conditions of “sealing” and “lubricity” have to be met at the same time. In recent years especially, there has been an even greater demand for lower friction in order to reduce mechanical loss while preventing leakage of a sealed fluid for environmental measures or the like. Lower friction can be achieved by generating dynamic pressure between sealing faces by rotation and letting the sealing faces slide in a state that a liquid film lies between the sealing faces, so-called bringing into a fluid lubrication state. However, since positive pressure is generated between the sealing faces in this case, a fluid flows to the outside of sealing faces from a positive pressure part. This corresponds to side leakage in a bearing and leakage in a case of sealing.
The viscosity of liquid is greater than that of gas and thus, in a case of the liquid sealing even between plain faces, a dynamic pressure effect can be obtained by a minute undulation, rough recesses and projections, or the like on the faces. Therefore, a structure giving priority to a sealing performance is often adopted. Meanwhile, several mechanisms having a pumping effect of bringing leaked liquid back to the high pressure side in order to obtain the sealing and the lubricity at the same time are proposed. For example, Patent Citation 1 discloses the invention in which a plurality of spiral grooves of moving a fluid to the high pressure chamber side is provided in circumferential direction on a seal face of a rotating ring (hereinafter, called as “Background Art 1.”)
As the invention relating to sliding parts, the present applicant has already filed the application of the invention in which a plurality of dimples 50 is provided on a sealing face S, a cavitation formation region 50a on the upstream side of each of the dimples 50 is arranged close to the low pressure fluid side, a positive pressure generation region 50b on the downstream side is arranged close to the high pressure fluid side, a fluid is suctioned in the cavitation formation region 50a on the upstream side, and the suctioned fluid is returned to the high pressure side from the positive pressure generation region 50b on the downstream side shown in FIG. 6 (hereinafter, called as “Background Art 2”. Refer to Patent Citation 2.)
However, in Background Art 1 described above, in a case where there is a pressure difference between inner and outer peripheries of the sealing face, a pumping operation against the pressure is required, and there is sometimes a case where the fluid cannot be pushed back depending on the magnitude of the pressure. Therefore, although leakage can be prevented in a case of a small pressure difference, there is a problem that a leakage amount is unavoidably high in a case of a large pressure difference.
Background Art 2 described above is the remarkable invention in a point that both functions of leakage prevention and lubrication can be exerted irrespective of the magnitude of the pressure difference between the inner and outer peripheries of the sealing face. However, since a basic shape of each of the dimples 50 is a crank shape, movement of the fluid from the cavitation formation region 50a on the upstream side to the positive pressure generation region 50b on the downstream side somewhat lacks smoothness. Thus, there is a fear that dynamic pressure generation on the low pressure fluid side X in the positive pressure generation region 50b becomes excessive and leads to leakage. Since a distance from a pressure peak position of the dynamic pressure generation region to the low pressure fluid side cannot be sufficiently large, there is a problem of a fear that the insufficient distance leads to leakage. Further, since the cavitation formation region on the upstream side is arranged close to the low pressure fluid side, radial width of the cavitation formation region cannot be increased. Thus, there is a problem that a negative pressure generation starting point cannot be increased.